REASSESSMENT is not always a good idea. Stockbrokers warn shares can go down as well as up. This also applies to artistic reputations, as the disappointing retrospective, William MacTaggart 1903-1981, at Edinburgh's Scottish National Gallery of Modern Art till May 10, demonstrates.

His pictures are mostly muddy, over-dense, duff, heavy-handed, their churned-up, thickly textured impasto surfaces oppressive in tone and sticky in paint quality. Only rarely does a rich gleam of stained-glass colour filter through the opaque sludge. More often black outlines and clumsy brushwork obscure the image.

Is this ham acting in the name of Expressionism? Certainly it's without any trace of the true emotional charge one cherishes in the deeply-felt work of his inspirational heroes, Munch and Rouault. While they lived life on a nerve-edge, MacTaggart was cushioned by bourgeois Edinburgh society. How could he experience their angst? No new territory was explored, he merely followed in the footsteps of others, diluting their experience as he went. So, how did he succeed? For one thing, he had the right name.

The name MacTaggart is so well known (even Lord Chancellor Derry Irvine has borrowed MacTaggarts for his London abode!) that it's almost synonymous with Scottish art. Grandson of the famous nineteenth-century landscape painter William McTaggart (a different spelling differentiates the two), young MacTaggart landed on his feet from the word go.

Affluent, cultured parents encouraged art as a ''worthwhile career''. The house was stuffed with pictures - not just by his grandfather but also by Boudin and other French Impressionists. At 14 his father built him a studio in the garden of their big family home at Loanhead, Midlothian. Bronchial illness meant MacTaggart, who lived to be 78, didn't go to school, so had plenty of time to sketch.

In 1919 he began attending Edinburgh College of Art part-time, but quit after two years. He was already exhibiting at the RSA, RGI, and SSA. In 1923 his doting mother took him to Italy and France for a year where he had his first exhibition - at Cannes. The death of both parents in 1930 provided the 27-year-old with financial security plus the family house. A flying start to a career which ended in multiple honours, presidencies, and, in 1961, a knighthood. Why?

''He wore robes exceedingly well,'' remembers one expert. ''He also had a very pushy wife, Fanny, who entertained in their 60ft reception room. Once you get your foot in one door . . .!'' Thus Fanny's salon helped oil the MacTaggart wheels.

MacTaggart's early work was traditionally sound if uninspired. Indeed his sketch books are some of the nicest things here: fresh charcoal drawings of trees and hills. His student friendship with Scottish contemporaries like Crozier and Gillies obviously helped him establish a context. As he often said later: ''We are all children of Cezanne.''

From the 1920s, MacTaggart divided his time between Scotland and France and the more vibrant pictures, like Gate in the Wood or Cassis 1927, were probably painted on the spot. Lucky chap to know picturesque villages like Bormes Les Mimosas above St Tropez in their pre-war bliss. Meanwhile, he visited Peploe in Antibes and St Raphael, where Anne Redpath of tartan repute - eight years his senior - lived.

In 1929 his first solo show, at Edinburgh's Aitken Dott, did well, with a rave account from friend Harvey Wood and pictures selling for four to 25 guineas. (Nowadays MacTaggart's work averages #5000 to #7000, with a good one fetching around #13,000.)

However, fate was to deal a useful twist in the shape of Fanny Aavatsmark, a well-connected Norwegian sent to the south of France as hostess for her consul uncle. She was also an aspiring art critic. She soon got engaged to MacTaggart's best friend, Harold Morton. He and MacTaggart often visited her in Oslo, which is how they became familiar with Munch's work, persuading the SSA to invite Munch to show in Edinburgh in 1931 - with controversial results.

Moreover, in 1937 Fanny had a change of heart, marrying MacTaggart in prosperous Morningside. The honeymoon was spent in Paris, Berlin, and Scandinavia. They then bought the Georgian townhouse at 4 Drummond Place where they lived for the next 40 years, when not travelling. Annual trips to France included the important 1952 Rouault show where MacTaggart was impressed by ''how he got one colour against another''.

MacTaggart's charm and pleasant disposition, his VIP friendships with Compton Mackenzie, Britten, Peter Pears et al, plus Fanny's push, served him well. Via these ordinary still lifes of poppies and rather lumpen landscapes, he gradually became the grand old man of Scottish art.

So what is MacTaggart's legacy? Undoubtedly the much over-used and abused term ''painterly'' - now equated with Scottish art - can partly be laid at his door.

As to long-term merit? As the Japanese would say, let history judge.